Many drivers say the large digital billboards flashing ads every few seconds along Bay Area freeways are just too bright and too distracting.

And they may be right.

A Swedish study published in the journal Traffic Injury Prevention concludes that digital billboards hold the gaze of drivers longer than two seconds. Previous studies have shown that anything that takes a driver's eyes off the road for longer than two seconds increases the risks of a crash.

"This study validates what is common sense when it comes to digital billboards," said a statement from Mary Tracy, president of Scenic America, a national nonprofit group that seeks to limit billboards. "Bright, constantly changing signs on the side of the road are meant to attract and keep the attention of drivers, and this study confirms that is exactly what they do."

The report will be presented to a national transportation conference in Washington, D.C., later this month and is sure to draw interest over the growing installation of these signs. Last month a three-judge panel ordered the removal of 100 digital billboards in Los Angeles, and Denver has banned them.

The Federal Highway Administration allowed digital signs for the first time in 2007 after concluding they did not pose a significant danger to drivers. But a follow-up report is pending and could be released this year.

California has no law banning the billboards and is one of 39 states that allows them.

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"We would need to review more research, so it's premature to call for a ban," said Jonathan Adkins, executive director of the Governors Highway Safety Association. "There is a role for digital messaging such as that employed by states to convey Amber Alerts and other safety messages."

Caltrans has considered using digital ads on its electronic freeway signs as a way to raise more cash. And revenue is why more billboards are being installed in cities strapped for cash such as San Jose.

There are more than 1,800 digital billboards nationwide, more than double the number five years ago. In the Bay Area, there are digital advertising signs on Highway 101 near Great America, IKEA in East Palo Alto and in Redwood City, on Interstate 880 near Marina Boulevard, on Highway 237 near First Street, on I-80 east of the Benicia Bridge and on Highway 85 at Almaden Expressway. Another is planned at the 280-880 interchange adjacent to the Valley Fair shopping center.

While there is no data that indicates an increase in accidents caused by the Bay Area signs, many drivers are opposed to them.

"Monstrosities" is what Merlin Dorfman of San Jose calls them, while insurance agent David Whitlock says he has found them a major distraction.

"The brightness is by far too bright for at night," he says. "When the advertisement switches from a brighter color to a darker color, your eyes cannot adjust fast enough and you end up losing vision of the roadway."

Officials with sign companies could not be reached for comment, but Bryan Parker, an executive vice president for Clear Channel Outdoor, told USA Today last year that "there's no doubt in my mind that they are not a driving distraction."

Several years ago, a study by the Virginia Tech Transportation Institute concluded the signs did not pose a danger, but its findings have been challenged by critics.

The Federal Highway Administration requires states to regulate the distance between signs and how long one image can remain on screen before changing to another.

Last summer many South Bay motorists howled when a digital sign was installed off Highway 85 at Almaden Expressway. But criticism eased when the brightness was reduced.

"I still don't like the sign," said Marge White of San Jose, who says she frequently sees drivers ahead unexpectedly slowing on the freeway and guesses they may be reading the ads. "But it's not as distracting since it's not as bright as before."